These teachings relate generally to detectors of electromagnetic radiation and, more specifically, relate to focal plane arrays (FPAs) of infrared (IR) radiation detectors and to detectors responsive to other spectral bands, including the visible band.
Referring to FIG. 1 it can be seen that conventional imaging sensor optics do not uniformly illuminate a flat image plane 1 that contains a two dimensional array having rows and columns of detector pixels. When a circular entrance aperture 2A that is made through a cold/light shield structure 2B is illuminated by a uniform scene irradiance, the relative intensity of the energy falling on a unit area of the image plane 1 is given by the fourth power of cosine xcex8, where xcex8 is an angle referenced to the optical axis 3 of the imaging sensor optics. As can be appreciated, the gain of the individual light detecting pixels located at the image plane 1 becomes an important consideration in order to compensate for the nonuniformity of the scene illumination across the image plane 1. Note that the focal length (fl) is shown as the distance from the entrance aperture 2A to the image plane 1.
One previous optical shading gain correction technique employs a voltage mode amplifier, typically a charge to voltage converter, that has fixed gain values set by adjusting the size of a capacitor or a resistor in the feedback path of an operational amplifier. While this technique works well for one-dimensional scanning-type image sensor arrays, it does not provide the required gain compensation in the two axes (x and y) of a staring-type image sensor, when the charge to voltage conversion is not performed in the detector""s input amplifier. While it may be possible to extend the previous column-based gain correction technique to two dimensions, practical circuit limitations have made this approach difficult to implement.
Reference with regard to a column-based circuit chain that produces a signal with a corrective gain value can be made to U.S. Pat. No.: 6,288,387, Black et al., xe2x80x9cApparatus and Method for Performing Optical Signal Intensity Correction in Electro-Optical Sensor Arraysxe2x80x9d.
The cos4xcex8 induced shading gain illustrated in FIG. 1 has been manageable for relatively small format image sensors (e.g., 256xc3x97256 pixels) and below. However, the trend towards larger format image sensors, such as 640xc3x97480 and 1 kx 1 k and larger, has made the shading-induced gain nonuniformity problem more severe, and has significantly complicated the ability to compensate for the shading-induced gain nonuniformity.
It should be noted that the undesirable effects of the optical shading may be removed from the output electrical signal using nonuniformity correction digital circuitry. However, since the correction is performed after the analog output signal experiences analog-to-digital conversion, the signal to noise ratio (SNR) is degraded for those pixels that are located away from the center of the two dimensional array. This is, therefore, not a preferred technique for dealing with the optical shading problem.
It should be appreciated that the optical shading need not be limit ed to only cos4xcex8 induced shading from a circular aperture. In fact, it can be any arbitrary shading caused by optical aberrations.
The foregoing and other problems are overcome, and other advantages are realized, in accordance with the presently preferred embodiments of these teachings.
In accordance with a first aspect of these teachings there is provided a radiation sensor that includes sensor optics having an entrance aperture and a two dimensional array of unit cells spaced away from the entrance aperture. Each unit cell includes a radiation detector having an output coupled to an input of a gain element, and each gain element is constructed so as to have at least one component with a value selected to set the gain of the gain element to a value that is a function of the unit cell""s location along an x-axis and along a y-axis within the two dimensional array. In this manner a compensation is made for a variation of scene illumination across the two dimensional array, where the variation in scene illumination results from a relative intensity of the energy impinging on a unit area of the two dimensional array that varies by the fourth power of cosine xcex8, where xcex8 is an angle referenced to an optical axis of the radiation sensor.
In one embodiment the gain element includes a direct injection (DI) amplifier circuit that includes an integration capacitance Cint and a sample and hold capacitance Cs/h,where the value of one or both of Cint and Cs/h is selected to set the gain of the DI amplifier circuit to a value that is a function of the unit cell""s location within the two dimensional array of unit cells.
In a second embodiment the gain element includes a source follower per detector (SFD) amplifier circuit having an integration capacitance Cint, where the value of Cint is selected to set the gain of the SFD amplifier circuit to a value that is a function of the unit cell""s location within the two dimensional array of unit cells.
In a further embodiment the gain element includes a charge transimpedance amplifier (CTIA) circuit that includes an integration capacitance Cint connected in a feedback path of the CTIA circuit, where the value of Cint is selected to set the gain of the CTIA circuit to a value that is a function of the unit cell""s location within the two dimensional array of unit cells.
In accordance with a further aspect of these teachings, where the two dimensional array of unit cells is arranged in y rows and x columns, the radiation sensor further includes a plurality of column readout circuits individual ones of which are switchably coupled to individual ones of the unit cells of one of the columns of unit cells. Each of the column readout circuits is constructed to have a first stage having a gain element with a least one component that has a value selected to set the gain of the gain element to a value that is a function of the location of the column within the two dimensional array, and is further constructed to include a second stage: having an input coupled to the first stage. The second stage includes a gain element having a gain that is varied as a function of a location within the two dimensional array of a row of unit cells that is being readout. In this case a combination of the gain of the first stage and the gain of the second stage compensates for a variation of scene illumination across the two dimensional array, where the variation in scene illumination results from the relative intensity of the energy impinging on the unit area of the two dimensional array that varies by the fourth power of cosine xcex8.
In a presently preferred embodiment the first stage gain element includes a charge transimpedance amplifier (CTIA) circuit that includes a feedback capacitance Cfb connected in a feedback path of the CTIA circuit, and the second stage gain element includes a differential Voltage Controlled Current Source (VCCS) that converts a voltage output of the first stage CTIA to a current. The value of the feedback capacitance Cfb is set according to the formula:
Cfbi=Cfbocos(atan((|(x/2)xe2x88x92i+0.5|)/fl)),
where I=column number;
fl=distance from the aperture to the two dimensional array;
Cfbo=feedback capacitance with no illumination nonuniformity compensation; and
x=the total number of columns;
and where the second stage differential VCCS current gain Gm is given by the formula:
Gmk=(Ggmo)/cos(atan((|(y/2)xe2x88x92k+O.5|)/fl)),
where k=row number;
fl=distance from the aperture to the two dimensional array;
Ggmo=voltage-current gain with no illumination nonuniformity compensation; and
y=the total number of rows.
The differential VCCS may include a current output DAC that is digitally programmed as a function of a row address being readout.